The invention relates to a method for operating a fuel system of an internal combustion engine, with the fuel system including a fuel tank and at least one ventilation device having a vent valve, for ventilating the fuel tank. The invention further relates to a fuel system.
Methods of the afore-mentioned type are known in the art. The respective fuel system is, for example, associated to a motor vehicle or a drive system of the motor vehicle. The drive system has hereby in particular at least one internal combustion engine and is configured, for example, as a hybrid drive system, i.e. has the internal combustion engine and at least one electric machine, with the internal combustion engine and the electric machine jointly generating a drive torque of the drive system at least temporarily. The internal combustion engine is supplied by the fuel system with fuel from the fuel tank. A volatile hydrocarbon fuel, for example gasoline, is oftentimes used as fuel. The fuel tank thus normally contains both a volume of liquid fuel and also a volume of gaseous fuel which accumulates in particular above the liquid fuel. The fuel tank may be a closed tank, in particular a pressure tank, or a partially closed, particularly also unpressurized, tank. The closed tank is used in particular to reduce emissions.
Due to fluctuations in the temperature of the fuel, caused for example by changes in the ambient temperature, pressure fluctuations may occur in the fuel tank. For this reason, the fuel tank is provided with a ventilation device to provide ventilation of the fuel tank so that excess pressure in the fuel tank can be reduced by the ventilation device. For this purpose, the ventilation device vents the fuel tank, for example via a vent line. During ventilation, both gaseous and liquid fuels can escape from the fuel tank through the ventilation device or vent line. The vented fuel thus is initially present as a mixture of gaseous and liquid fuels. This is particularly the case when ventilation of the fuel tank is carried out at high fuel tank internal pressure. In this case, high flow rates of vented fuel are encountered as a result of the high pressure or great pressure differential between the fuel tank internal pressure and the pressure outside of the fuel tank so that liquid fuel is entrained by the gaseous fuel.
The gaseous fuel may readily be supplied to the internal combustion engine or its intake system, and a fuel accumulator, preferably designed as a charcoal canister, can be arranged between the fuel tank and the internal combustion engine and associated to the ventilation device. The fuel accumulator is provided to temporarily store, i.e. receive, gaseous fuel, when gaseous fuel that is not needed is present and is to be fed back as soon as the gaseous fuel can be discharged into the internal combustion engine. However, liquid fuel must not migrate into the fuel accumulator or the internal combustion engine to avoid damage or malfunction.
For that reason, the ventilation device may have at least one separator, which is used for separation of gaseous and liquid fuels. The separator is thus provided to prevent transfer of liquid fuel from the fuel tank through the ventilation device into the internal combustion engine or fuel accumulator. The separator separates hereby liquid fuel and allows passage of gaseous fuel. The separated liquid fuel passes into a buffer of the separator. The term buffer is hereby not to be understood as the presence of an actual (temporary) storage of liquid fuel. Rather, liquid fuel can be discharged directly from the buffer or the separator, preferably in the direction of the fuel tank. This may, however, result in a rise of the fill level of the buffer, for example, when limiting the discharged volume flow, especially through a line cross-section or the like. The separated liquid fuel may thus at least temporarily not be expelled fast enough as it is introduced into the buffer. Of course, there is, however, the possibility to temporarily store the liquid fuel, for example for a specific period of time.
During operation of the fuel system, the amount of liquid fuel present in the buffer or separator should be prevented from exceeding a limit amount, i.e. the fill level of the buffer thus exceeds a limit fill level, as this can adversely affect the effectiveness of the separator. The risk that liquid fuel escapes again with gaseous fuel from the separator and is entrained in the direction of the fuel accumulator or internal combustion engine increases as the amount of liquid fuel in the buffer increases. For this reason, the fuel delivery device is associated to the separator and is used to convey liquid fuel from the buffer, in particular in the direction of the fuel tank. The fuel delivery device is usually designed as a suction jet pump, with fuel being frequently used as operating agent of the suction jet fuel and conveyed by a fuel pump of the fuel system from the fuel tank in the direction of the internal combustion engine.
In the known methods for operating the fuel system of the internal combustion engine, the fuel tank is normally ventilated by the ventilation device as a result of the fuel tank internal pressure and a temperature. This means that the vent valve of the ventilation device is set as a function of the fuel tank internal pressure and the temperature for ventilating the fuel tank. However, this is only possible when the internal combustion engine is activated, because only in this case is a control unit, associated to the internal combustion engine or the fuel system, being activated. This control unit is used to adjust the vent valve based on the particularly measured fuel tank internal pressure and the particularly measured temperature for ventilating the fuel tank. At standstill phases, i.e. when the internal combustion engine and thus the control unit are deactivated, the fuel tank internal pressure is normally limited by at least one mechanical overflow valve which is dimensioned for a typical pressure level. When the temperature changes in the fuel tank, for example from heat input by the internal combustion engine still warm from operation or by external influences, a fuel tank internal pressure adjusts in the fuel tank in correlation with the vapor pressure of the fuel. When the fuel tank internal pressure reaches or exceeds the typical pressure level and especially a defined maximum fuel tank internal pressure, the overflow valve opens to vent the fuel tank. A deactivated internal combustion is hereby to be understood as an internal combustion engine at a standstill whereas an activated internal combustion engine is to be understood as at least idling, in particular, as providing a torque.
When the internal combustion engine is deactivated, the fuel tank is normally vented exclusively by the fuel tank internal pressure. However, it is oftentimes the case that the permissible maximum fuel tank internal pressure varies with temperature, i.e. is therefore dependent there from. Especially when the internal combustion engine is deactivated and the fuel tank internal pressure can only decreased via the overflow valve, there may be a situation in which the fuel tank is subject to a fuel tank internal pressure which causes forces that exceed a strength of the fuel tank. This leads, for example, to unacceptable flow behavior or to unwanted, irreversible deformations of the fuel tank, in particular of a fuel tank shell.